FOM: Length of proofs and consistency of formal systems by Prikh

Harvey Friedman 13 Sep 1998 10:11:50 +0100 wrote:
>> Sazonov 5:32PM 8/31/98 writes:
>> >The resulting formal system proves to be
> >*feasibly consistent* in the sense that there exists no formal
> >proof of feasible length (say, proof written in a book) which
> >leads to a contradiction. It is Rohit Parikh who introduced (in
> >his paper in JSL, 1971) this, still rather NEW AND UNFORTUNATELY
> >COMPLETELY UNEXPLORED IN F.O.M. UNDERSTANDING OF CONSISTENCY OF
> >A FORMAL SYSTEM.
>> The FOM may be interested in the new Handbook of Proof Theory, editor Sam
> Buss, that has just come out from North-Holland. There is an extensive
> article there by Pudlak on the lengths of proofs, including my early result
> which I called "finite Godel's theorem" concerning how many steps are
> needed to prove in T that T has no inconsistency of size <= n.
Yes, there is (very important!) complexity theoretic aspect in
the traditional technical sense of estimating the length of
proofs (either of a contradiction in a theory or of some its
restricted Consis statement, etc.).
However, the paper of Parikh contains *additionally* new and
very simple idea that we could restrict ourselves only to those
formal proofs which are physically existing. Proofs which exist
only in our imagination because of their unrealistic length (say
10^{10^10}) are not considered as proper proofs at all. (What
would we think about a mathematician which asserts that he has
an imaginary proof of imaginary length on imaginary paper sheets
resolving a difficult mathematical problem? We will ask him to
present a *real* proof with all the necessary details!) This may
serve as a realistic and reasonable approach to f.o.m. Note,
that there exists no precise borderline between physically
existing (in principle) and imaginary proofs. Of course this
approach changes the notion of consistency of a formal system
and is not reducible to (however is essentially based on) the
complexity theoretic length-of-proof aspect.
If we admit Goedel completeness theorem as a plausible *informal
postulate* saying that any (even feasibly/physically) consistent
formal theory has a meaning (model, interpretation) then
considering this new class of theories may extend mathematics
radically by new concepts which have no direct counterparts in
ZFC universe.
For example, it may be obtained in this way a very unusual
version of the concept of continuum satisfying unexpected
(but reasonbable) properties. I wrote about this in some my
postings to FOM. It is the task of f.o.m. to construct
formalisms for any kind of fundamental concepts which pretend
to be mathematical. In some cases only feasibly consistent
(but potentially, say, in 10^{10^10} number of steps
inconsistent) formalisms may work.
Vladimir Sazonov
-- | Tel. +7-08535-98945 (Inst.),
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